class Url:
    """
    Class to build the python challenge url with the solution string and an
    optional slug.
    """

    def __init__(self):
        self._url = ''
        self._slug = 'html'
        self._solution = ''

    BASE_URL = 'http://www.pythonchallenge.com/pc/def'

    @property
    def url(self):
        """
        Gets the url string of the builder.
        :return: url string
        """
        return f'{self.BASE_URL}/{self.solution}.{self.slug}'

    @property
    def solution(self):
        """
        Returns the solution string
        :return: solution string
        """
        return self._solution

    @solution.setter
    def solution(self, solution):
        """
        Sets the solution string
        :param solution: str
        """
        self._solution = solution

    @property
    def slug(self):
        """
        Returns the slug string
        :return: slug string
        """
        return self._slug

    @slug.setter
    def slug(self, slug):
        """
        Sets the slug string
        :param slug: str
        :return: slug string
        """
        self._slug = slug